A conventional multiple split optical waveguide 31 having a tree configuration includes a plurality of stages of Y branch parts formed in a substrate each Y branch part splitting an incident light beam evenly into two parts. An incident light beam is received from one end of the substrate, and a plurality of split light beams exit from the other end of the substrate. In such an optical waveguide, as shown in FIG. 4, the central axial line of the incident light beam of the Y branch part 32 of the first stage is in parallel with those of the Y branch parts of the following stages. Thereby, the waveguide paths are all provided with a same length.
The waveguide path which is split by each Y branch part is brought back into a parallel direction before reaching the Y branch part of the subsequent stage. Because increasing the curvature of the waveguide path causes an increase in the transmission loss, the waveguide paths are required to be curved without involving a part having any large curvature. This causes the length of each waveguide path to be increased, and therefore the size of the substrate to be increased. This problem gets particularly pronounced as the number of stages of Y branch parts increases. In particular, as the number of stages increases, the waveguide paths which are split near the base end of the substrate or the upstream part of the waveguide paths are required to be separated from each other by a greater distance, and this contributes to the increase in the length of each waveguide path and the size of the substrate.
As shown in FIG. 5, it is also conceivable to reduce the distance between adjacent waveguide paths or increase the density of the waveguide paths in a multiple split optical waveguide 41 having a tree configuration so that the waveguide paths may be branched out at a relatively short distance. The branch waveguide paths split by the Y branch parts of the last stage are curved before reaching the output ports on the other end of the substrate. Because the waveguide paths are curved only after being split by the Y branch parts of the last stage, the length of each waveguide path can be made shorter than those shown in FIG. 4, but the loss increases and the unevenness in the lengths of the waveguide paths causes variations in the outputs between different output ports.
According to the proposal disclosed in Japanese patent No. 3,030,108, the central axial line of the incident light of the Y branch part of the first stage is not in parallel with those of the second and subsequent stages, and the angles formed between the central axial line of the incident light beam of the Y branch part of the first stage and those of the outer most branch parts of the third and any subsequent stages are made progressively greater toward the later stages. This arrangement eliminates the need to excessively curve the waveguide paths, and the length of each waveguide path to be reduced without involving any significant loss due to large curvatures between the Y branch parts of adjacent stages.
However, according to the arrangement disclosed in Japanese patent No. 3,030,108, the outer waveguide paths tend to spread apart from adjacent waveguide paths toward the later stages, and this creates the need to significantly curve the waveguide paths to put the waveguide paths which are spread apart from each other toward each other. Therefore, as the number of stages of Y branch parts increases, the loss due to the curvature of the outer waveguide paths may increase. If the curvature of the outer waveguide paths is kept small, the length of each waveguide path increases, and the size of the substrate increases. The spreading apart of the outer waveguide paths causes the length of each waveguide path to vary between the inner and outer waveguide paths, and this not only increases the loss but also increases the variations of loss between different waveguide paths.